The present application claims priority of Chinese patent application Serial No. 200610011945.9, filed May 19, 2006, the content of which is hereby incorporated by reference in its entirety.
The existing cargo inspection system based on radiographic imaging generally causes a single energy ray to interact with the object under inspection, and then detects the ray having penetrated through the object under inspection to obtain an image. Although such a system can reflect the change in shape and mass thickness (i.e., mass per unit area obtained by multiplying a thickness t by a density) of the object under inspection, it can't discriminate the material of the object under inspection.
The dual-energy detection method for distinguishing the material attributes of objects has been proposed for a long time as disclosed in U.S. Pat. No. 5,044,002, and within low energy region, the method has been widely applied to various fields such as osteoporosis diagnosis, geographic oil layer detection and material discrimination for small objects. However, it has long been recognized that within high energy region (>1 MeV) the slight difference caused by electron pair effect is insufficient to implement material discrimination, thereby having a poor practicability.
In 1990s, U.S. Pat. No. 5,524,133 disclosed that the angular distribution of Compton scatter effect and the isotropy of electron pair effect were used to analyze the scatter components of X-rays caused by each effect after the X-rays interacted with an object, thereby discriminating the substance's atomic number of the object interacting with the X-rays. In U.S. Pat. No. 5,524,133, high-energy X-rays is caused to interact with a target having a greater atomic number after interacting with the object. Then several detectors are positioned at different angles with respect to the target so as to detect Compton scatter effect and electron pair effect. Nevertheless, since it is very difficult to detect the scatter after interaction between the target and the X-rays, which have penetrated through the object, a large incident dose of X-rays is usually required. In addition, the SNR (signal to noise ratio) of detection signals is rather low because the detector array arranged at such angles within the same horizontal plane is susceptible to the interference from its neighbor channel. The above disadvantages have an adverse impact on the determination of the substance's atomic number, and the image quality is unsatisfactory. Thus this method has not been put into practical application since proposed in 1993.
Later in U.S. Pat. No. 6,069,936 and international application WO 00/43760 a high-energy radiation source is employed to generate X-rays, which are filtered by means of specific materials to obtain another ray having a higher energy spectrum. The penetrating X-rays having two energy spectra are detected after they interact with the substance. The substance's atomic number and material type are then determined by computing the ratio between the two detection values.
During the X-rays having two energy spectra interact with the inspected object in this method, as the inspected object grows in thickness, the two energy spectra of the X-rays, which have penetrated through the inspected object, have an ever-decreasing difference and rapidly become identical to each other. In this case, it can't discriminate the inspected object any more.